Why are women the only primates with enlarged breasts?

We love exploring the traits that set humans apart from other primates and animals. For example, our bipedal posture, hairless bodies, higher encephalization, rich cumulative culture, and language. Yet, we often overlook one of the most conspicuous differences: the development of prominent breasts in women at puberty, something that does not happen in other primates. The intriguing question is — why?

Desmond Morris tackled this question in his 1967 seminal book, The Naked Ape. His ideas became fixed in the collective imagination like a scab that was difficult to peel off, as they were innovative, striking, intuitive and easy to understand. But they were also pure speculation, and subsequent research has chipped away at their foundations.

Morris proposed that female breasts evolved alongside bipedalism, as they resembled buttocks. In quadruped primates, sexual signals are most visible on the backside. In contrast, in the upright position of humans, most interactions are face-to-face, and the buttocks are not as visible, so why not bring them to the chest? If men were attracted to buttocks, Morris argued, it was better to have two sets: one in front, and one behind.

Other researchers later suggested that breasts could be attractive as they serve as indicators of sexual maturity. Whether due to Morris’s hypothesis or this one, the idea of breasts evolving through sexual selection has been the most widely discussed hypothesis. Consequently, much research has focused on male perspectives regarding breast attractiveness. And the results are very varied.

Some studies suggest men prefer medium and large breasts — though not excessively large, as they may lack firmness. Others claim men favor smaller breasts. Men like some breasts more than others, depending on cultural factors. Poor men prefer larger breasts than rich men. Men do not care about the size of the breasts. Men in Mali believe that being sexually attracted to breasts is perverted. The sheer variability of findings could fill an entire article.

In 2021, Bogusław Pawłowski and Agnieszka Zelazniewicz from the University of Wroclaw in Poland published a literature review criticizing the theory that breasts evolved through sexual selection. Their review argued that it’s unlikely enlarged breasts were initially deemed attractive, as in primates they are typically more pronounced during periods of lower fertility: lactation and pregnancy.

The researchers also find it strange that such an arbitrary trait evolved simply to attract men. In nature, male selection is often based on characteristics that represent a direct benefit to the individual, such as willingness to mate, health or reproductive potential. In contrast, there is evidence that breast size does not correlate with any of these characteristics. In fact, women begin to develop breasts before they are even fertile.

Pawłowski and Zelazniewicz acknowledge the obvious role of breasts in human sexuality, but believe this was not their original evolutionary purpose. Often in evolution, traits that arise for one function are co-opted for another, a process known as exaptation. They suggest that the female breast may have initially been a byproduct of increased subcutaneous fat in humans.

Most studies on the development of primate breasts have been carried out on macaques, because of their usefulness as models for studying breast cancer. In these animals, the mammary glands also develop during puberty, but their volume barely increases. Therefore, women’s breasts are different not because they develop earlier, but because they accumulate a greater amount of fat. In fact, while there is considerable variability, a positive correlation exists between breast size and body fat index.

Molecular and archaeological data suggest that, approximately 2 million years ago, Homo ergaster began to increase its level of subcutaneous fat. This helped newborns sustain energy for enhanced brain development and allowed adults to better adapt to colder climates. In women, the surge in estrogen during puberty further increased their fat levels, preparing them for pregnancy.

But why does fat accumulate more in breasts than elsewhere on the body? Fat distribution is related to the number of estrogen receptors. According to Pawłowski and Zelazniewicz, female chimpanzees, for instance, have higher concentrations of these receptors around the genitals, and in the uterus and breasts. It is therefore likely that this receptor distribution — already present in our common ancestor — predisposed humans to increased breast development.

But why is there a higher level of fat in the breasts than in other parts of the body? Fat distribution is related to the number of estrogen receptors. According to Pawłowski and Zelazniewicz, female chimpanzees have a higher density of these receptors around the genitals, in the uterus and on the breasts. It is therefore likely that our common ancestor already exhibited this distribution, which facilitated a greater increase in mammary size.

The accumulation of fat in the breasts and hips became a distinctive feature between men and women, which, according to Pawłowski and Zelazniewicz, may have subsequently driven their attractiveness. Sexual selection, they argue, would then have acted on breast traits, explaining why some women with low body fat still have large breasts, and vice versa. Indeed, genetic studies have even identified two loci (location of a gene on the chromosome) associated with breast size, independent of overall subcutaneous fat, though they account for only about 1% of size variation.

While more research is needed to validate Pawłowski and Zelazniewicz’s hypothesis, their explanation is grounded in evidence and represents progress over speculative theories made in the last century. Still, it is striking that discussions about the evolutionary origin of breasts barely mention their critical role in women’s sexual experiences.

The function of enlarged breasts extends beyond attraction, as proper stimulation during sex is pleasurable for most women. One study found that 80% experience arousal from breast stimulation, and some can achieve orgasm through this alone due to the neural connection between the nipples and uterus. There are also quite a few studies on how to regain nipple sensitivity after a mastectomy, given its importance in female sexuality.

Additionally, nipple stimulation — whether from a sexual partner or breastfeeding — triggers the release of oxytocin. Psychiatrist Larry Young posits that romantic love may have evolved as an adaptation of neural circuits originally designed for maternal bonding. Therefore, female breasts, through oxytocin release during sexual intercourse, may activate this bonding mechanism.

The origin of breasts’ allure may not stem so much from their size, shape, color or supposed resemblance to buttocks. Perhaps, it all began when a curious Homo ergaster with fuller breasts discovered with her partner that they were a wonderful way to achieve pleasure, desire and love.

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